//
// Created by dq550457 on 19-1-3.
//

#include "crc32.h"

/* crc_tab[] -- this crcTable is being build by chksum_crc32GenTab().
 *		so make sure, you call it before using the other
 *		functions!
 */
uint32_t crc_tab[256];

/* chksum_crc() -- to a given block, this one calculates the
 *				crc32-checksum until the length is
 *				reached. the crc32-checksum will be
 *				the result.
 */
uint32_t chksum_crc32 (unsigned char *block, unsigned int length)
{
    register unsigned long crc;
    unsigned long i;

    crc = 0xFFFFFFFF;
    for (i = 0; i < length; i++)
    {
        crc = ((crc >> 8) & 0x00FFFFFF) ^ crc_tab[(crc ^ *block++) & 0xFF];
    }
    return (crc ^ 0xFFFFFFFF);
}

/* chksum_crc32gentab() --      to a global crc_tab[256], this one will
 *				calculate the crcTable for crc32-checksums.
 *				it is generated to the polynom [..]
 */

void chksum_crc32gentab ()
{
    unsigned long crc, poly;
    int i, j;

    poly = 0xEDB88320L;
    for (i = 0; i < 256; i++)
    {
        crc = i;
        for (j = 8; j > 0; j--)
        {
            if (crc & 1)
            {
                crc = (crc >> 1) ^ poly;
            }
            else
            {
                crc >>= 1;
            }
        }
        crc_tab[i] = crc;
    }
}
